Basal-like breast cancers (BL-BC) are poor prognosis tumors for which no targeted therapies are available. A genome-wide siRNA screen identified proteasome addiction as an epigenetic dependency of poorly differentiated breast tumor cells arrested in a bipotent state, which are highly enriched in BL-BC and frequently retain tumor-initiating potential. BL-BC cell lines were commonly sensitive to the proteasome inhibitor drug bortezomib compared to normal mammary epithelium and well-differentiated breast cancer cell lines. Bortezomib effect in BL-BC was mediated by aberrant accumulation of the proapoptotic protein Noxa. However, why BL-BC are poised to Noxa accumulation and especially sensitive to Noxa-induced death is unclear. In this proposal we will dissect specific mechanisms behind Noxa-dependent response to proteasome inhibition in BL-BC. In the first aim we will define the contribution of Noxa in regulating therapeutic response to bortezomib in multiple BL-BC mouse models silenced for Noxa. In the second aim we will explore specific regulatory mechanisms that we suspect underlie aberrant Noxa accumulation in BL-BC, including transcription factors, chromatin modifications, E3 ubiquitin ligases and microRNAs. In the third aim we will employ siRNA screening technology and bioinformatics to identify the highest value functional factors regulating Noxa sensitivity in BL-BC, focusing on highly connected nodes within key biological networks. Together, these studies will shed light on Noxa-dependent mechanisms controlling BL-BC response to bortezomib, laying the foundation for new methods for predicting or augmenting drug response in these tumors. Moreover, these studies may open up new ways to activate Noxa-induced death in BL-BC without blocking proteasome function indiscriminately. Both the PI and the collaborators are accomplished investigators in the proposed areas of investigation. The PI is currently an instructor at Harvard Medical School whose goal is to establish an independent research program to dissect mechanisms of drug response in BL-BC by integrating cancer biology, functional genomics, bioinformatics and mouse models into a unique pipeline. This award will allow the PI to pursue advanced training in bioinformatics and collect preliminary data on a very well-defined mechanism of response to proteasome inhibitor drugs in BL-BC, thus enhancing his transition to independence.